Liquid crystal displays (LCDs) continue to improve in cost and performance, becoming a preferred display technology for many computer, instrumentation and entertainment applications. Typical LCD mobile phones, notebooks, and monitors comprise a light guide plate for receiving light from a light source and redistributing the light more or less uniformly across the LCD. Existing light guide plates are typically between 0.8 mm and 2 mm in thickness. The light guide plate must be sufficiently thick in order to couple effectively with the light source, typically a cold cathode fluorescent lamp (CCFL) or a plurality of light emitting diodes (LEDs), and redirect more light toward the viewer. Also, it is generally difficult and costly to make light guide plates at a thickness smaller than about 0.8 mm and a width or length greater than about 60 mm using the conventional injection molding process. On the other hand, it is generally desired to slim down the light guide plate in order to lower the overall thickness and weight of the LCD, especially as LEDs are becoming smaller in size. Thus, a balance must be struck between these conflicting requirements in order to achieve optimal light utilization efficiency, low manufacturing cost, thinness, and brightness. However, in many LCD and general illumination lighting systems with relatively large dimensions (typically greater than 300 mm diag.) there is a need for relatively thick light guide plates with thickness typically greater than 2 mm. This high thickness is often dictated by dimensional and mechanical rigidity requirements as well as by the larger size of LEDs best suited for these larger lighting systems.
The extrusion roll molding process disclosed in U.S. Pat. Pub. No. 2011/0242847 provides an effective means for producing thin light guide plates in a roll-to-roll fashion and at relatively high line speeds. These extrusion casting processes become ineffective when the thickness of the patterned light guide plate exceeds about 1 mm. At this higher thickness range, replication fidelity for the light extraction micro-pattern becomes very poor under typical process conditions and line speeds are very slow. In order to extend the efficiencies of the extrusion roll molding process to relatively thick light guide plates and other types of thick micro-patterned optical films there is a clear need to modify this process in a way that eliminates some of the problems in attaining good replication fidelity for the desired micro-pattern while maintaining relatively high line speeds and good production efficiency.
The method of choice heretofore has been the injection molding process and some variants thereof. In this process a hot polymer melt is injected at high speed and pressure into a mold cavity having micro-machined surfaces with patterns that are transferred onto the surfaces of the solidified molded plate during the mold filling and cooling stages. Injection molding technology is quite effective when the lateral dimensions (width and/or length) are relatively small (≦about 300 mm). However, for relatively large light guide plates, the injection molding process requires very large molds and significant levels of injection pressure which typically leads to poor replication and high residual stress and birefringence in the molded plate, creating poor dimensional stability and low production yields. Also, injection molding is a batch process and therefore quite inefficient in high volume operations. Another approach used to produce thick light-guide plates is to print a discrete (‘dot’) micro-pattern on one side of a flat, extruded cast sheet using ink-jet, screen printing or other types of printing methods. This process is disadvantaged in that the extrusion casting step requires an additional costly printing step and the shape and dimensions of the discrete micro-extractors are predetermined and not well-controlled and, therefore, light extraction and redirection is inefficient.
While there have been solutions proposed for preparing various types of light guide plates using relatively fast extrusion casting, roll-to-roll operations, these operations are limited to relatively thin light guide plates. Thus, for applications requiring relatively thick light guide plates for both the LCD backlight and general illumination markets, there remains a need to prepare cost-effectively light guide plates with a thickness greater than about 1 mm using an efficient single pass extrusion casting process.